This invention relates to corona-resistant wire enamel compositions and conductors insulated therewith.
Dielectric materials used as insulators for electrical conductors may fail as a result of corona occurring when the conductors and dielectrics are subjected to voltages above the corona starting voltage. This type of failure may occur for example in certain electric motor applications. Corona induced failure is particularly likely when the insulator material is a solid organic polymer. Improved dielectric materials having resistance to corona discharge-induced deterioration would therefore be highly desirable. For some applications, mica-based insulation systems have been used as a solution to the problem, whereby corona resistance is offered by the mica. Because of the poor physical properties inherent in mica, however, this solution has been less than ideal because of the relatively large amount of space that the mica based compositions require.
Solid, corona-resistant dielectric materials are particularly needed for high-voltage apparatus having open spaces in which corona discharges can occur. This is especially true when the space is over approximately 1 mil in thickness and is located between the conductor and the dielectric, or when there is a void located in the dielectric material itself. The service life of the dielectric is much shorter when these gaps or spaces are present.
Resins containing a minor amount of an organometallic compound of either silicon, germanium, tin, lead, phosphorus, arsenic, antimony, bismuth, iron, ruthenium or nickel are disclosed by McKeown (U.S. Pat. No. 3,577,346) as having improved corona resistance. Corona lives of up to four hundred times that of polymers without the organometallic additive are disclosed.
A composition having anti-corona properties is disclosed by DiGiulio et al, in U.S. Pat. No. 3,228,883, to consist of a mixture of ethylene-alpha-olefin copolymer, a homo- or copolymer covulcanizable therewith and a nonhydroscopic mineral filler, such as zinc, iron, aluminum or silicon oxide. However, there is no appreciation whatsoever in this patent that the use of submicron-sized alumina or silica particles is necessary to achieve significant improvement in corona resistance.
A molded epoxy resin composition which contains hydrated alumina and silica is disclosed by Linson, in U.S. Pat. No. 3,645,899, as having good weathering and erosion resistance, but appears to have no particular resistance to corona breakdown.
Polyethylene resin with various fillers, including alumina and silica, appears to be disclosed in U.S. Pat. No. 2,888,424 issued May 26, 1959 to Precopio et al. But again, there is no concern or appreciation of corona-resistant properties; the fillers, including such counter-productive materials for corona properties as carbon black, are added only to improve mechanical properties.
Thus, there is a continuing need for corona-resistant materials which are easily fabricated for use as electrical insulation and a further need for additives which can convert dielectric materials susceptible to corona damage to corona-resistant materials. Accordingly, it is the principal object of the present invention to provide a corona-resistant resin, useful in various electrical insulation forms to satisfy these long-felt needs.